Guess #1:

http://support.microsoft.com/?kbid=308360
OFF2000: Error Messages Caused by Nimda Virus When You Open or Save
Files in Office Programs

Guess #2:
Something has broken that option.
You can reset the toolbar to see if that helps.
tools|customize|toolbars tab
select worksheet menubar
click reset

With #2, you'll have to re-do any customizations you've done to that toolbar.

dan wrote:

When I select "Open" from the "File" menu in Excel, I get only a beep, no
file selection window appears. All other function in Excel seem to work.

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Dave Peterson